Cross-flow filtration wherein a fluid is caused to flow parallel to the upsteam side of the filter surface has been used for many applications where a quasi-steady operation is desirable. With cross-flow filtration the build up of the solids on the upstream surface of the filter medium is inhibited by the shear exerted by the flowing fluid. This type of filtration has been largely restricted to microfiltration and membrane filtration.
In order to use filtration to separate entrained particulates of less than a predetermined size from entrained particles of a larger size it is necessary to maintain the pore size of the filter medium constant. With throughflow filters wherein the fluid flow is normal to the surface of the filter medium, the filter cake effectively decreases the pore size of the filter medium wherefore the maximum particle size which passes through the filter medium increases from a maximum value when a filter cycle is initiated to a minimum value when the filter cycle is terminated for filter cake removal. While the build up of a filter cake is inhibited and in some cases entirely prevented when cross-flow filtration is employed, some of the fine particles which will not entirely pass through the filter medium do enter the wall matrix of the filter medium and remain within the medium. As a consequence, the maximum particle size which will pass through the filter medium in a cross-flow filter also decreases as the filter cycle continues.